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Vetterlein, F, et al. xe2x80x9cMethod for measurement of heart rate in awake, non-instrumented small animalsxe2x80x9d, American Journal of Physiology 247(16), pp. H1010-H1012, 1984.
www.indusinstruments.com; description of ECG monitoring device for mice with built-in ECG electrode contact pads.
The present invention relates generally to physiologically monitoring devices for animals, and is particularly directed to noninvasively recording of the cardiac electrocardiogram in conscious mice. The invention will be specifically disclosed in connection with an apparatus and method that permits uninhibited movement of rodents on a platform that limits the range of motion of the animal without constraint, and conductive pads arranged to promote automatic recording of the electrocardiogram of the mouse while freely moving on said platform.
Animals in general, and rodents in particular, have long been used in biomedical research of human disease conditions and therapeutics. The mouse is probably the most extensively used animal in biomedical research. Mice are the animal of choice for experimentation because of their small size, short reproductive cycle, and breadth of knowledge known about mice and their genetics. As completion of the human and mouse genome mapping projects near completion, non-invasive measurement of physiological parameters in large numbers of mice should become desirable. For example, measurement of heart rate, heart rate variability, and electrocardiographic indices have, for nearly a century, provided clinicians with important diagnostic tools; these data in mice may provide valuable information regarding the roles of genes and drugs in human disease.
A problem exists with currently available techniques for recording the electrocardiogram in mice. Currently available techniques require anesthetic and/or surgical implantation of radiotransmitters. Anesthesia, however, may depress cardiovascular function, and recovery after radio transmitter implantation is nearly 3 weeks. Neither anesthetic nor surgery is amenable to studying large populations of mice. In studying or understanding disease processes, it is almost always preferable to examine a subject in a conscious and relaxed state. If large numbers of conscious mice could be examined, then investigators might be able to determine more precisely gender, strain, and age dependent effects. Moreover, the effects of genes or drugs can be interpreted more precisely if the confounding effects of anesthetic and surgery are eliminated.
One popular method of recording ECGs in conscious mice is surgical implantation of radiotransmitters into the belly of the mouse. However, the implants weigh nearly 2 gms and impose a significant strain on the animal""s physiology. In mice, it takes nearly 3 weeks for the animal to recover from surgery. While the method does permit collection of ECG data in conscious free moving mice once it recovers from the original surgical insult, the surgery and recovery period does not advance recording ECGs in large quantities of mice.
Various literatures describe the use of radiotelemetry for recording the ECG in small rodents, including mice. One perceived benefit of the radiotransmitter implants is that the device does enable the recording of the ECG in the conscious mouse. However, these publications also point out the anesthesia and surgical procedures and the lengthy recovery period.
Vetterlein et al. (Am J Physiol 247:H1010-H1012; 1984) describe a method for measurement of heart rate in awake, noninstrumented rats. In their paper, they describe detection of the heart rate in a rat by placing the rat in a small enclosure within a plastic 4-sided cage with built-in metal plates. A manual switch was activated to record heart rate when it was determined that a front leg and a back leg were touching two pads.
There is yet need for an apparatus and method to non-invasively monitor and record ECGs in large quantities of conscious mice that do not entail anesthetic, surgery, or confinement.
Accordingly, we developed a novel method and apparatus for obtaining ECGs in conscious mice by placing the animal on a platform embedded with paw-sized ECG pads connected to signal recording equipment. The cardiac electric potential of the animal is detected via the paws of the animal and relayed to an amplifier and signal acquisition equipment. This technique is much less traumatic to the mouse, requires no anesthesia or surgery, attachment of wires to the animal or confinement of the animal, and promotes rapid screening of large numbers of mice.
It is one object of the invention to provide a device that measures the ECG in conscious mice by way of an array of paw-sized conductive pads arranged in a platform upon which a mouse is free to move.
Applicant has recognized that most mice will not attempt to alight from a raised platform. Were the array of conductive pads arranged on a platform simply placed on a plane of a larger surface, the animal will in most instances transpose itself from the device to the neighboring surface. With the pads embedded in an elevated platform, however, the animal is least likely to depart said platform. Accordingly, it is another object of the invention to provide an elevated platform configured to limit the range of motion of the animal, yet not restrain the animal, which enables measurement of the ECG and other physiological parameters in conscious mice.
Applicant has recognized the need for evaluating large numbers of mice. Mice have highly developed sense of smell. Moreover, they defecate and urinate often. Accordingly, it is desirable to enable the device of the present invention to record ECGs in numerous mice by providing a new platform for each mouse. Hence, the device of the present invention provides a disposable conductive-pad embedded platform such that ECGs for each mouse are obtained via a new and fresh platform.
As such, a further aim of the present invention is the provision for rapid and facile attachment and detachment of the conductive-pad embedded platform into a housing that provides electrical contact between the platform and the ECG recording equipment.
Applicant has also recognized that mice seldom stand still and, as such, it is unlikely that the contact between paws and conductive pads will be continuous. Rather than record electrical data that is not representative of the animal""s electrocardiogram, it is desirable to cease recording when the animal""s paws are not in cooperation with the pads and to activate recording when the animal""s paws are in contact with the pads. One object of the invention is to provide means for selectively recording the ECG when 3 of the animals"" paws are in contact with 3 conductive pads. It is yet another object of the present invention to automate measurement and recording of the ECG dependent on cooperation between the animal""s paws and the conductive pads.
Noninvasive measurement of other physiological variables in mice, as well as other means for obtaining those measurements, is becoming increasingly valuable to investigators using mouse models. For example, nuclear magnetic resonance (NMR) imaging is increasingly valuable because of its ability to non-invasively image the internal organs of the mouse. However, the animal is typically anesthetized to keep the animal from moving, and introduction of metallic instrumentation may confound data measurements inside the magnet, or test environment. To date, no one has described NMR imaging of the conscious mouse, and the ECG in the unconscious mouse is used to gate the acquisition of the images. Accordingly, in another aspect of the present invention, none of the components in the invention are metallic, and the electrical conduits comprise of ionic solution-filled tubing linking the animal and ECG platform positioned within the test environment to the recording equipment positioned external to the test environment. As such, one embodiment of the invention we describe enables ECG-gated NMR imaging of conscious mice.
Mice exhibit behavior suggestive of stress if they are placed in a receptacle of small volume and enclosing; walls. Moreover, mice will attempt to climb walls and may succeed if the walls are insufficiently high or if made from a material that the mouse is able to sufficiently grip with its paws. Accordingly, the device of the present invention does not include enclosing walls and so does not substantially invoke behavior suggestive of confinement nor give the animal a vertically arranged object or surface to inspect or climb. Rather, the present invention provides a recording platform that maximizes the likelihood of achieving effective quiescence of the animal, while enabling the recording of electrocardiographic signals from the conscious, unrestrained animal.
Additional objects, advantages, and other novel features of the invention will be set forth in part in the description that follows, and in part, will become apparent to those skilled in the art upon examination of the following or may be learned with the practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.